The distribution system in North America is such that domestic premises are fed with a 2-phase (180.degree. phase relationship) utility supply, the local transformer centre tap giving an artificial Neutral for normal low-current loads at 115 V, while the voltage across phases is 230 V for power loads such as air-conditioning, motor drives and heaters. The local transformer primary is usually fed from an overhead fused 25 KV supply, so that the contactor switch contacts must safely withstand any reasonable short-circuit fault on the load side of the meter.
Known contactor designs exist for performing such switching functions in association with domestic electricity meters used in North America.
In U.S. Pat. No. 4,388,535 the feed connections are provided with sets of fixed pairs of contacts, and related sets of sprung, contacted shorting bars are positioned in proximity to the fixed contact sets, such that when they are actuated the two switch sets make contact, connecting the feed or utility side to the domestic load side.
Actuation is achieved by a moving plunger within a power solenoid coil, and a set of pivoted bellcrank levers operate to push open the sprung shorting bars or to retract to close them, the spring forces providing the necessary contact closure. A microswitch is used to interrupt the solenoid coil drive during the OPEN and CLOSE actuation functions, ensuring that the energisation is only momentary, thus preventing the coil from over-heating and possible burn-out.
In U.S. Pat. No. 4,430,579 the construction is similar to U.S. Pat. No. 4,338,535, using sprung contacted shorting bar switch sets to create the 2-pole contactor function. But the actuation method adopted is different in that the solenoid is double-acting, the plunger being naturally attracted centrally into a power drive coil when energised, this being the point of greatest flux concentration. In being attracted centrally, the plunger is dynamically over-driven past its centre to mechanically latch at each end of its stroke. The coil power is typically 2,000 W for a reliable double-action mechanical latching function.
This solenoid double-action is used to translate the switching function via suitably guided roller-aided push rods, either to CLOSE or OPEN the two sprung switch sets, the contact closure force being provided by the compression springs behind each shorting bar. In order to ensure that the contacts do not separate under short-circuit fault conditions, a relatively high force must be applied by each compression spring.
The solenoid plunger is profiled in such a way as to perform both the translation and mechanical latching functions simultaneously. A variant of the profiled plunger uses a similarly profiled, hardened steel plate suitably pinned to the plunger, to perform the same mechanical translation and latching functions, respectively. A microswitch is again used to interrupt the solenoid coil drive to prevent the coil from over-heating.
It is an object of the present invention to provide an improved two-pole contactor.